Too many males may have been the reason the dinosaurs died out 65 million years ago, say Leeds University scientists.

They believe the dinosaurs could have been like modern-day reptiles such as crocodiles whose sex depends upon the temperature before they are born.

The idea is that the asteroid which struck changed the world's climate, causing it to be cooler and leading to the birth of a preponderance of males.

The male-female imbalance would have led to the dinosaurs' extinction.

Sun block

Although some scientists think dinosaurs were on their way out anyway before the space rock hit, most experts believe one or more asteroid impacts probably triggered a series of global changes that killed off the great beasts - and great many other species of life, too.

It is thought the impacts would have kicked up dust that cooled the air. If dramatic volcanic activity was triggered by the event as well, even more dust and ash could have been thrown into the atmosphere to block out the Sun.

All this would have been bad news for the dinosaurs because it is well known that those at the top of the evolutionary pile are especially vulnerable to ecological changes.

Even those beasts that survived this great calamity and managed to eke out a meagre living might then have struggled to find a mate, according to the Leeds theory.

Different ways

No one really knows whether dinosaurs were much like other reptiles, or whether in some respects they resembled animal groups such as mammals.

Reptiles have a different type of metabolism to mammals and have various ways of determining the sex of their offspring.

In mammals, if a newborn has an X and a Y chromosome, it will be male; and if it gets two X chromosomes, it will be female, with a few, very rare exceptions. Similar mechanisms work for birds, snakes and some reptiles such as lizards.

But in crocodilians, turtles and some fish, the temperature at which eggs are incubated can affect the sex of the developing foetus.

David Miller, of the University of Leeds, and colleagues ran an analysis that showed a temperature shift could theoretically have led to a preponderance of males among the dinosaurs.

Other studies have shown that when there are too few females, eventually the population will die out.

Huge casualties

"The Earth did not become so toxic that life died out 65 million years ago; the temperature just changed, and these great beasts had not evolved a genetic mechanism (like our Y chromosome) to cope with that," said Dr Sherman Silber, an infertility expert in St Louis, who worked on the study.

But crocodiles and turtles had already evolved at the time of the great extinction at the end of the Cretaceous Period. So, how did they survive?

"These animals live at the intersection of aquatic and terrestrial environments, in estuarine waters and river beds, which might have afforded some protection against the more extreme effects of environmental change, hence giving them more time to adapt," the researchers say.

But other scientists are not convinced by the sex idea.

"More than 50% of all species that lived prior to the mass extinction were wiped out. In fact, the dinosaurs were not among the most numerous of the casualties - the worst hit organisms were those in the oceans," said Benny Pieser, of Liverpool John Moores University.

"I am afraid sex-selection mechanisms are an unlikely cause for the termination of the age of dinosaurs - despite the sexed-up headlines."